The papermaking industry has for some time needed a better way to enhance the wet strength of paper products. The commercial importance of paper products such as paper board, fine paper, newsprint, tissue and towel has fueled a need for improved compositions and methods that enhance the wet strength of paper products.
Known information offers limited choices having technical and economic disadvantages. It is known that carboxymethylcellulose, for instance, can be used to promote the wet strength imparting capacity of polyamide resins. However, the use of carboxymethylcellulose has several disadvantages. For instance, carboxymethylcellulose is a dry material, which makes it difficult to work with and requires special make-down equipment. Carboxymethylcellulose often requires applications at significant dosages. Also, carboxymethylcellulose can be an explosion hazard under certain conditions, and thereby can be a hazardous and dangerous material.
U.S. Pat. No. 3,049,469 teaches adding dilute aqueous solutions of a cationic resin and a water-soluble, carboxyl-containing material (an acrylic dry strength additive) to a dilute aqueous suspension of a paper pulp. The patent broadly teaches that sheeting and drying the pulp forms a paper product that exhibits enhanced dry and wet strength properties. The patent also broadly teaches that the improvement in wet strength is greater than would be expected from the combined action of the ingredients, thus indicating a synergistic effect when the two components are used together.
Unfortunately, the teachings of U.S. Pat. No. 3,049,469 are so broad and general that in describing suitable carboxyl-containing materials, the patent does not emphasize which features, if any, of carboxyl-containing materials may critically affect their performance. The single example provided by the patent does not indicate the molecular weight or the charge of the acrylamide-acrylic acid copolymer that is mentioned. The patent does not provide any guidelines about which carboxyl-containing materials may be unsuitable. The patent does not provide any guidelines about how the molecular weight of anionic polymers and the charge properties of anionic polymers may affect the performance of wet strength agents.
Huaiyo et al., Study of the Co-Use Technology of Polyamide Polyamine Epichlorohydrin Resin with Anionic Polymer to Kraft Reed Pulp Zhongguo Zaozhi (1997), 16(1), pp. 34-38 discloses in part that a polyamide polyamine epichlorohydrin resin used in combination with a polyacrylamide having a molecular weight of more than five million daltons can improve dry and wet strength of paper. Huaiyo, however, does not provide any guidelines about how the molecular weight and the charge properties of anionic polymers may affect the performance of wet strength agents. The high molecular weight polymers disclosed by the article are commercially disadvantageous. Such high molecular weight polymers, for instance, flocculate the sheets causing poor formation of paper. Also, it is known that when a polymer having such a high a molecular weight is used in solution, the solution must have impractically low solids contents in order to maintain acceptable flow properties.
The above-mentioned deficiencies and disadvantages are typical in the literature. Indeed, the art is replete with information that does not provide meaningful guidelines about which features, if any, of carboxyl-containing materials are critical, in imparting wet strength to paper products. The literature does not provide any meaningful guidelines that would enable an artisan to develop a method that enhances the wet strength-enhancing properties of a cationic strength agent without requiring increased amounts of materials.
For the foregoing reasons, there is a need for better methods to enhance the wet strength of paper products.
For the foregoing reasons, there is a need for improved compositions for making paper products having enhanced wet strength.
For the foregoing reasons, there is a need for compositions and methods that can promote the wet strength-enhancing properties of a cationic strength agent without requiring increased amounts of the wet strength agent or the carboxyl-containing material.